1. Field of the Invention
The present invention relates to solar cells and more particularly to current carrying bus configurations used to improve the efficiency of solar cells.
Current collection from the illuminated surface of a solar cell is conventionally provided by a solder-dip formed grid having collection grid lines extending across a majority of the cell's surface. A number of alternate patterns may be used. Typically, one or more bus lines interconnect the grid and form a common collection point for connecting the cell to an external circuit. The proper design of the collection grid and the accompanying bus is important to the solar cell's efficiency.
This design requires that the photosensitive surface area covered by metal, and hence shadowed from incident light, be small. On the other hand, the conductivity of the grid and bus(es) must be sufficiently high to minimize power (I.sup.2 R) dissipation losses in the collecting network. For any metal sheet conductance, there will be an optimum metal pattern for which the sum of the shadowing and I.sup.2 R losses are minimal. This represents the most favorable grid/bus pattern for that solar cell. Increasing the sheet conductivity of the bus metal allows the width of the bus(es) to be decreased without increasing the I.sup.2 R dissipation loss, and hence represents a more nearly optimum collecting network. In theory, the sheet conductivity can be increased by either increasing the metal thickness, or by employing a metal having a higher intrinsic conductivity. Increasing the metal thickness can be achieved up to a point, but then becomes impractical either because of constraints imposed by the metallization method, or by mismatch in the expansion coefficients of the metal and the underlying solar cell.
A common obstacle to obtaining high sheet conductivity collection grids and busses is the requirement that the thermal coefficients of expansion for the metal used in forming the network and the underlying cell be sufficiently compatible. For instance, single crystal silicon solar cells have linear thermal coefficients of expansion of about 2.times.10.sup.-6 .degree. C.-1 for temperatures ranging from 0.degree. C. to 50.degree. C. and about 2.4.times.10.sup.-6 .degree. C..sup.-1 for temperatures ranging from 50.degree. C. to about 100.degree. C. Typical low cost grid and bus forming techniques utilize lead-tin solder patterns to form the grid/bus network on the cell's surface. Alternatively, screened-on conducting inks are employed, or evaporated metal patterns are used. Lead-tin solders have very high expansion coefficients, typically 20-25.times.10.sup.-6 .degree. C..sup.-1, but are relatively ductile at room temperature. When the solder is cooled below room temperature, however, the solder becomes more rigid and the stress rises. The thicker the solder layer, the more serious the mismatch in expansion coefficient becomes upon thermal cycling.
In the formation of a solar cell by a solder dipping procedure, the thickness of the solder differs from the grid lines to the bus lines, and (if the bus is tapered) from one end of the bus to the other. This solder height is determined in part by surface tension in the solder, and in part by the width of the metal line to which the solder adheres. In a typical solar cell, the thickness of the solar on the bus pattern may be 3 to 5 times the thickness of the solder on the grid line. Accordingly, it is one object of this invention to reduce the solder thickness on the bus pattern without increasing the resistance of the bus. It is another object of this invention to provide a high conductivity metal bus with minimal solder thickness, while retaining solder as the metallization for the grid lines. In this manner, the cells are better able to withstand temperature excursions, particularly to temperatures of -45.degree. C. and below.
The electrical resistivity of lead-tin solders is typically about 10 to 15 times that of copper, depending on the solder composition. For equal thicknesses, the copper will therefore have a sheet conductivity about 10 to 15 times higher than that of the solder. Accordingly, the width of a copper bus can be decreased by 10 to 15 times if its thickness is equal to that of a solder bus, without a change in the I.sup.2 R dissipation of the bus and with a substantial gain in the photosensitive area on the surface of the solar cell. Several other common metals, including gold, and silver, also have electrical conductivity about equal to, or better than that of copper.
Pure copper has a linear thermal coefficient of expansion of about 16.6.times.10.sup.-6 .degree. C..sup.-1, which is substantially larger than that of the silicon in the solar cell. Furthermore, the copper used in conductors is typically much stiffer than the lead-tin solder at room temperature. Attaching a copper bus directly to an underlying solder metallized bus pattern on a solar cell along the entire bus length generally leads to peeling of the copper or solder from the silicon surface during thermal cycling. In the past, this problem has been overcome by laminating copper with a low expansion metal alloy such as Invar to provide a "sandwich" having an average expansion coefficient closely matching that of silicon. This approach adds substantially to the cost of the bus bars to be attached to the solar cell. Furthermore, the Invar materials are relatively poor electrical conductors and, for adequate electrical conductivity of the bus, substantial total bus metal thicknesses are required. Accordingly, the expansion coefficients must be very closely matched to avoid delamination with these thick bus materials. Furthermore, shadowing by the bus at other than vertically incident light increases.
The present invention avoids these problems, yet provides an increased electrical conductivity bus for current collection in a solar cell while permitting minimal solder thickness, (or other expansion coefficient mismatched metal bus thickness,) on the solar cell surface. The increased sheet conductivity for the bus permits redesign of the bus pattern and provides for increased output from a given solar cell wafer. The invention further provides for inter-cell electrical connections needed for assembly of cells into "strings." It also further provides a spacer between the front surface of the solar cell and a superstrate such as glass, which may have advantage when, for instance, silicone elastomers are used as pottants. Since the expansion coefficient of silicon and glass differ, it is important to separate the silicon from the glass by a predetermined amount in the finished module so that the difference in expansion can be accommodated in the soft elastomer layer which bonds the cells to the glass.